fix: address code review findings

- Fix duplicate Telegram notifications: SyncArtistDiscography no longer wipes
  notifications_sent for the whole artist on every cache-miss re-sync; only
  markers for releases that disappear are pruned (FK-safe via INSERT OR REPLACE
  + rgid NOT IN (...)).
- Cache empty MusicBrainz discographies via a new artist_settings.last_synced
  column (migration 009) so zero-release artists honor the TTL instead of being
  re-fetched every cycle.
- Wire the Web UI server and Telegram notifier scheduler into main.run/NewApp.
- Guard startPeriodicSync against overlapping syncs with a done-channel slot.
- Add server.public_url config; NewServerWithConfig derives reachable links
  and no longer advertises the 0.0.0.0 bind address.
- Web handlers: use scanner.ScanArtist per artist, drop always-false
  releaseIgnored lookup and dead endsWith, thread configured threshold.
- Limit :memory: DB pool to one connection so migrations and queries share the
  same in-memory store.

internal/database/database.go

@@ -25,10 +25,21 @@ func New(dbPath string) (*DB, error) {
 	// EVERY connection in the pool. A one-off "PRAGMA foreign_keys=ON" executed
 	// on the pooled *sql.DB only applies to the first connection and is lost on
 	// connections opened later by the pool, silently disabling the safety net.
+	// A plain ":memory:" database is private to the connection that opened it,
+	// so a pool of N connections would give N separate empty databases and
+	// migrations would appear missing on some. Limiting the pool to a single
+	// connection keeps one in-memory database per New() call, which is correct
+	// for both tests (isolated) and the single-process production service.
 	conn, err := sql.Open("sqlite3", dbPath+"?_foreign_keys=on")
 	if err != nil {
 		return nil, fmt.Errorf("open database: %w", err)
 	}
+	if dbPath == ":memory:" {
+		conn.SetMaxOpenConns(1)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("open database: %w", err)
+	}
 
 	// Enable WAL mode for better concurrent read performance.
 	if _, err := conn.Exec("PRAGMA journal_mode=WAL"); err != nil {
@@ -135,6 +146,10 @@ func (db *DB) migrate() error {
 			name: "008_add_mbid_to_artist_settings",
 			sql:  `ALTER TABLE artist_settings ADD COLUMN mbid TEXT;`,
 		},
+		{
+			name: "009_add_last_synced_to_artist_settings",
+			sql:  `ALTER TABLE artist_settings ADD COLUMN last_synced DATETIME;`,
+		},
 	}
 
 	for _, m := range migrations {
@@ -181,12 +196,13 @@ func (db *DB) isMigrationApplied(name string) (bool, error) {
 
 // ArtistSettings represents a row in the artist_settings table.
 type ArtistSettings struct {
-	ID                 string `json:"id"`
-	Name               string `json:"name"`
-	MBID               string `json:"mbid"`
-	IgnoreSingles      bool   `json:"ignore_singles"`
-	IgnoreCompilations bool   `json:"ignore_compilations"`
-	Monitored          bool   `json:"monitored"`
+	ID                 string    `json:"id"`
+	Name               string    `json:"name"`
+	MBID               string    `json:"mbid"`
+	IgnoreSingles      bool      `json:"ignore_singles"`
+	IgnoreCompilations bool      `json:"ignore_compilations"`
+	Monitored          bool      `json:"monitored"`
+	LastSynced         time.Time `json:"last_synced"`
 }
 
 // LocalAlbum represents a row in the local_albums table.